The present invention relates to a new surface coating or painting method which gives coherent paint and/or protective coatings, which can be given very good resistance to both chemical and mechanical damage. Although the method in question has primarily been conceived for use in connection with painting, it can also be used for a number of other purposes, for instance for permanently bonding grains or powdered material to a carrier of one kind or another.
In its general form, the method according to the invention can primarily be considered to constitute a new and entirely superior variant of the so-called powder paint procedure, according to the original concept of which, fine particles of a polymer which is not film forming at room temperature are applied to the surface which is to be coated, after which the paint particles are heated to a temperature well above the minimum film formation temperature (MFT) of the polymer, whereby the polymer particles are caused to be fused together to form a coherent layer. The heating can then take place after the particles have been applied to the surface in question, alternately the particles can be applied to a surface that has already been heated. Combinations of these two variants are also conceivable. In addition to applying the polymer particles in a dry condition to the surface in question, through e.g. so-called electrostatic coating, it has also previously been proposed to apply them in the form of a dispersion, the liquid component of which in such a case must be evaporated before the melting of the particles commences. These so-called dispersion paints have the advantage compared with other variants of powder paint that they are easier and cheaper to produce. A polymer dispersion with an appropriate particle size can be produced in a comparatively simple way by a concentrated polymer paint being charged into water under intensive stirring, the size of the particles then being determined by the recipe, the design of the equipment, the method of conveying the paint to the water, and the stirring speed. It is also easy to modify the water content subsequently in such a dispersion paint.
An advantage of the conventional powder paints that has previously been considered to be of great value is that these, at least in their original variants, are entirely free from solvent. According to a variant subsequently proposed, however, a minor quantity of solvent is added, in order to reduce the film formation temperature of the polymers.
In the U.S. Pat. No. 3,565,665, a further variant is proposed, according to which the surface which is to be protected is first coated with a porous layer containing particles of an appropriate polymer material, after which this layer is subjected to a solvent treatment which causes the particles to be fused together. It is proposed to provide the original porous layer by means of electrostatic coating, or by previously providing the surface which is to be coated with a sticky coat of glue.
Each of the above-mentioned methods has advantages and disadvantages, and the purpose of the present invention is to combine all of the favourable properties of these methods, and at the same time to eliminate as many of the earlier disadvantages as possible.
The hot film forming powder paints can thus be entirely free from solvent, but special methods, e.g. electrostatic coating, are required in order to apply them, at the same time as the heat resisting properties of the base limits the choice of conceivable polymers, as a satisfactory film formation requires heating well above the MFT of the polymer. In practice, the choice is thus limited to polymers with a low MFT value which, in turn, involves comparatively soft polymers, which usually do not have sufficient chemical and mechanical properties, and sometimes have a tendency towards sticking if the surfaces are painted and these are placed in contact with each other. When the method according to the invention is used, however, there is no limitation to polymers with a certain MFT. There is no need to apply the polymer material in question by means of expensive procedures of the type electrostatic coating, or through special separately applied bonding agents either, in this case.